The intestinal fatty acid binding protein (IFABP) is present in humans in two forms which differ by a alanine to threonine substitution at amino acid 54. In vitro, the threonine form (Thr54) is characterized by a two- fold higher binding affinity for long chain fatty acids than the alanine form (Ala54). The Thr54 form also is associated with higher plasma insulin concentrations and insulin resistance in the Pima Indians. This project examines whether the in vitro binding differences are physiologically significant. Net dietary fatty acid absorption, the kinetics of appearance of dietary fatty acids into plasma, and the time course of lipid oxidation will be determined following a standard test meal incorporating [l-13C]-palmitic acid as a tracer of fatty acid metabolism in healthy subjects homozygous for either the Thr54 form (n=20) or the Ala54 form of IFABP (n=20). These results will be related to rates of 24-hour substrate oxidation (measured in the respiratory chamber).